1. Field of the Invention
This invention relates to a method of preparing a bleached phase hologram and a bleaching composition used in the method.
2. Description of the Prior Art
Various photosensitive materials are known which can be used in holography, including silver halide photographic materials such as those based on silver halides, dichromated gelatin, or photopolymerizable materials. Of these materials silver halide photographic materials are most widely used because of their high photographic speed, their sensitivity to a broad range of spectral wavelengths and the capability of producing holograms with a very high diffraction efficiency.
In order to prepare a hologram by using a silver halide photographic material, the material is exposed to an interference beam formed by superimposing an object beam with a reference beam, each beam from a laser. Then the exposed material is developed and fixed to form an amplitude hologram based on the developed silver. Since such an amplitude hologram has a low diffraction efficiency, theoretically predicted to be around 7%, the amplitude hologram is usually further processed, i.e., bleached (rendered transparent) to be converted into a phase hologram, which is practically used. Various bleaching compositions for amplitude holograms have been reported, for example, in Applied Optics, 8, 88 (1969) and ibid., 9, 1367 (1970), however, none of these known compositions satisfy all of the requirements for holograms, (i) diffraction efficiency, (ii) stability to light, (iii) light scattering property, (iv) light absorption, (v) spacial frequency, etc. Since a bleached phase hologram essentially has a high diffraction efficiency, the urgent problem is how to improve the stability to light. Of conventional methods of preparing a bleached phase hologram, a most preferable method from the standpoint of stability to light comprises bleaching an amplitude hologram in any conventional known manner as disclosed in the above references, and then immersing the bleached hologram into an aqueous potassium iodide solution so as to convert the relatively unstable silver compound which has been formed by the bleaching and is readily decomposable by light into a fairly stable silver iodide. Although this method can certainly be used to produce a better hologram with an improved stability to light than other known methods, impurities still tend to be formed and remain in the hologram because a complete conversion of the unstable silver compound into silver iodide is impossible using this method. These impurities function as nuclei for printing out of silver, thus promoting the darkening of the hologram upon exposure to light. Another drawback of the above-cited method is the crystal growth accompanying the conversion of silver compounds into silver iodide, which increases the light scattering of the resulting hologram.